The invention relates to electronic copying/printing systems and more particularly to an electronic multi-color copying/printing system and non-interactive development systems therefor.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. The areas of charge dissipated on the photoreceptor correspond to residual or background voltage levels. Thus, the photoreceptor may, in a digital printer, contain two voltage levels while in a light/lens machine there is a vast array of levels.
This latent charge pattern is rendered visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Conventional xerographic imaging techniques which were directed to monochrome image formation have been extended to the creation of color images including highlight color images. In one method of highlight color imaging, the images are created using a raster output scanner to form tri-level images including a pair of image areas and a background area intermediate the two image areas.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as in the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.CAD or V.sub.ddp). V.sub.ddp is the voltage on the photoreceptor due to the loss of voltage while the photoreceptor remains charged in the absence of light, otherwise known as dark decay. The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.DAD or Vc (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background area is exposed such as to reduce the photoreceptor potential to halfway between the V.sub.CAD and V.sub.DAD potentials, (typically -500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.CAD than Vwhite (about -600 volts), and the DAD developer system is biased about -100 volts closer to V.sub.DAD than V.sub.white (about 400 volts). As will be appreciated, the toners used in a system such as described above need not comprise different colors but may have other distinguishing characteristics. For example, both toners could be black but one toner may be magnetic and the other non-magnetic.
The above described tri-level imaging process is utilized in the 4850.TM. printer. This printer is capable of creating images with black toner and one of several highlighting colors. Since the printer can only have two, one black and one color, developer housings installed at any one time, changing color from job to job typically requires recharging the color development system with a new, different color developer material having toner of the proper color or replacing the whole development system with one already charged with developer having toner of the desired color. Color changes from job to job are time consuming for the "recharge" technique and the use of multiple development subsystems requires storage space.
Non-interactive development systems are know U.S. patent application Ser. No. 07/619,649 filed on Nov. 29, 1990 in the name of Grace T. Brewington and assigned to the same assignee as the instant application discloses color image creation using a plurality of non-interactive developer structures wherein the color is user selectable. Selection of a desired color establishes the duty cycle of each of a plurality of developer structures which, in turn, determines how much, if any, of each color toner is deposited on a particular image. The duty cycle of each developer structure may vary between zero and a predetermined maximum time which corresponds to the time it takes for an image area on a charge retentive surface to move through a development nip intermediate the charge retentive surface and one of the developer structures.
A number of U.S. patents which appear to be relevant to various aspects of the present invention disclosed herein are discussed below.
U.S. Pat. No. 5,012,299 granted to Sawamura et al on Apr. 30, 1991 discloses a color adjustment apparatus for a color copying machine including a touch-key for inputting color adjustment data which causes the values for exposure and main-charge outputs and development bias to be varied in accordance with a correction to be made.
U.S. Pat. No. 4,546,722 granted on Oct. 15, 1985 to Toda et al discloses a development apparatus having a toner carrying member and a piezoelectric vibrator for displacing toner from the toner carrying member and causing it to fly in a manner to avoid depositing toner onto a non-image area of an image bearing surface. Such an arrangement prevents degradation of the charged image for the purpose of image preservation. The apparatus avoids adverse influences upon the electrostatic latent image so as not to cause disturbance in the resulting image if applied in a multiple copy per exposure process to produce a plurality of copies. This apparatus is non-interactive from a latent electrostatic image preservation standpoint, but does not appear to be non-interactive from a developed toner image standpoint, and therefore, would seem to allow unwanted scavenging of multi-colored toner to occur. This apparatus seems to be designed to prevent degradation of the charged image for the purpose of latent image preservation and not for the purpose of preventing degradation of the toned image pattern.
U.S. Pat. No. 4,833,503 granted to Christopher Snelling on May 23, 1989 relates to a multi-color printer using a sonic toner release development system to provide either partial or full color copies with minimal degradation of developed toner patterns by subsequent over-development with additional colors and minimal back contamination of developer materials. Multiple scanning beams, each modulated in accordance with distinct color image signals, are scanned across the printer's photoreceptor at relatively widely separated points, there being buffer means provided to control timing of the different color image signals to assure registration of the color images with one another. Each color image is developed prior to scanning of the photoreceptor by the next succeeding beam. After developing of the last color image, the composite color image is transferred to a copy sheet. Development is accomplished by vibrating the surface of a toner carrying member thereby reducing the net force of adhesion of toner to the surface of the toner carrying member. By appropriately limiting the magnitude of vibration of the toner carrying member in this development system toner will be released from the surface only in those areas in proximity to image areas where toner deposition is actually desired. Thus, selective clouds of toner may be made to occur only in correspondence with those image areas that are actually to be developed by toner deposition onto them.
U.S. Pat. No. 4,987,456 granted to Snelling et al on Jan. 22, 1991 relates to a resonator suitable for generating vibratory energy which is arranged in line contact with the back side of a charge retentive member bearing an image on a surface thereof, in an electrophotographic device, to uniformly apply vibratory energy to the charge retentive member. The resonator comprises a vacuum producing element, a vibrating member, and a seal arrangement. Where the vibratory energy is to be applied to the charge retentive surface, a vacuum is applied by the vacuum producing element to draw the surface into intimate engagement with the vibrating member, and edge seal arrangement. The invention has application to a transfer station of enhancing electrostatic transfer of toner from the charge retentive surface to a copy sheet, and to a cleaning station, where mechanical vibration of the surface will improve the release of residual toner remaining after transfer.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a donor roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon, a recording electrode and a signal source connected thereto, for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information, a plurality of mutually insulated electrodes provided on the developing roller and extending therefrom in one direction, an AC and a DC source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller thereby forming the toner particles into smoke in the vicinity of the donor roller and the sheet.
U.S. Pat. No. 5,010,367 granted to Dan A. Hays on Apr. 23, 1991 relates to a non-interactive or scavengeless development system for use in color imaging. To control the developability of lines and the degree of interaction between the toner and receiver, an AC voltage is applied between a donor roll and electrodes supported adjacent to the surface of said donor roll to enable efficient detachment of toner from the donor to form a toner cloud. An AC voltage applied between the donor roll assembly and an image receiver serves to position the cloud in close proximity to the image receiver for optimum development of lines and solid areas without scavenging a previously toned image.
With those non-interactive development systems discussed above that create uniform powder clouds in the development zone it is difficult to measure and to define the appropriate developer bias to optimize development. This is because toner clouds create a space charge in the development zone which affects the desired value of voltage bias applied to development systems. The space charge must be taken into account when setting the developer bias. Thus, the magnitude of the space charge, which can not be accurately determined, adversely affects the developer bias.